leetcode 42 Trapping Rain Water

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Given n non-negative integers representing an elevation map where the width of each bar is 1, compute how much water it is able to trap after raining.

For example,
Given [0,1,0,2,1,0,1,3,2,1,2,1], return 6.

class Solution {public:    typedef vector<int>::size_type sz;    int trap(vector<int>& height) {        sz size = height.size();        if (size <= 2) return 0;        int ret = 0; // how many units of rain water trapped        // the max left height        sz max_lo;        if (height[0] > height[1])            max_lo = 0;        else            max_lo = 1;        for (sz i = 2; i < size; ++i) {            // 当height[i] > height[i - 1] 时才可以蓄水            if (height[i - 1] < height[i]) {                 int base = height[i - 1];                // for 循环是因为 height[i] 可能与多栏蓄水如 【4 2 0 3】                // 当 j 小于 max_lo 时说明左边没有更高的栏所以不用计算                for (int j = i - 2; j >= int(max_lo); --j){                    int h = 0;                    // 当 height[j] > base 时说明该栏与height[i] 之间可以蓄水                    if (height[j] > base) {                        h = min(height[j], height[i]);                          ret += (i - j  - 1) * (h - base);                        base = h;                    }                    if (height[j] >= height[i])                        break;                }                if (height[i] >= height[max_lo]) {                    max_lo = i;                }             }        }        return ret;      }};

参考后

class Solution {public:    typedef vector<int>::size_type sz;    int trap(vector<int>& height) {        sz size = height.size();        if (size == 0) return 0;        sz lo = 0;        sz hi = size - 1;        int ret = 0;        while (lo < hi) {            while (height[lo] <= height[hi]) {                sz i = lo + 1;                 while (height[i] < height[lo]) {                    ret += height[lo] - height[i];                    cout << ret << "i" << endl;                    ++i;                }                lo = i;                if (lo == hi) return ret;            }            while (height[lo] > height[hi]) {                sz i = hi - 1;                while (height[i] < height[hi]) {                    ret += height[hi] - height[i];                    --i;                }                hi = i;                if (lo == hi) return ret;            }        }        return ret;    }};